Force-fitting shift elements are frequently used in the drive trains of motor vehicles, for example in automatic transmissions. Force-fitting shift elements, such as multi-disk clutches or multi-disk brakes, are characterized by a variable torque transfer capability. An overlapping, simultaneous opening of a first force-fitting shift element and closing a second force-fitting shift element enables, for example, a power shifting of an automatic transmission. Such shift elements are frequently closed by a hydraulic closing pressure acting on a piston against a spring force or against a counter-force acting on a different side of the piston. The closing pressure acting on the piston is predetermined by a control valve. For this purpose, a control unit sends a control signal to the control valve, which, depending on the control signal, by setting the valve lift, transmits the system pressure of a hydraulic circuit to the piston, or reduces such pressure. If the control valve is fully open, apart from any possible flow losses and leakage losses, the full system pressure applies at the piston, which leads to the closing of the shift element. If the control valve is fully closed, apart from leakage at the valve, no closing pressure applies at the piston. If the shift element is preloaded by a spring in its open position, the spring force thereby leads to the opening of the shift element.
Patent application DE 42 40 621 A1 of the applicant discloses a method for controlling and regulating load transfer for an automatic transmission. Thereby, an overlapping shift is described, whereas, after the shift is concluded, the engaging clutch is driven to the maximum pressure.
Patent application EP 0 783 079 A1 describes a method for the shift control of an automatic transmission. Therein, at the end of the shift, the valve allocated to the shift element to be closed is open, by which the maximum pressure is applied at the closing piston, in order to fully close the shift element.
The driving of the engaging shift element to the maximum pressure serves the purpose of, among other things, flushing the hydraulic valve that is allocated to the shift element. Thereby, the depositing of contaminants present in the hydraulic oil at, for example, the control edges of the hydraulic valve, can be avoided.
However, through the full opening of the hydraulic valve, the system pressure prevailing in the hydraulic circuit is applied at the piston, reduced by any possible flow or leakage losses. However, in certain individual cases, this high closing pressure for the shift element is not necessary at all to securely transfer the torque to be transferred by the shift element. As such, the shift element must be accordingly designed for this closing pressure, although the closing pressure required to perform the function would be lower. Due to the higher mechanical stress, both the component weight and the need for installation space of the shift element increase. However, under certain circumstances, other users of the hydraulic circuit require a high system pressure, such that a reduction in system pressure is not possible.